Msm type photodetection device with resonant cavity comprising a mirror with a network of metallic electrodes

ABSTRACT

This invention relates to an MSM type photo-detection device designed to detect incident light and comprising reflecting means ( 2 ) superposed on a support ( 1 ), to form a first mirror for a Fabry-Pérot type resonant cavity, a layer of material ( 3 ) that does not absorb light, an active layer ( 4 ) made of a semiconducting material absorbing incident light and a network ( 5 ) of polarization electrodes collecting the detected signal. The electrodes network is arranged on the active layer and is composed of parallel conducting strips at a uniform spacing at a period less than the wavelength of incident light, the electrodes network forming a second mirror for the resonant cavity, the optical characteristics of this second mirror being determined by the geometric dimensions of the said conducting strips. The distance separating the first mirror from the second mirror is determined to obtain a Fabry-Pérot type resonance for incident light between these two mirrors.

TECHNICAL DOMAIN

This invention relates to an MSM type photo-detection device withresonant cavity comprising a mirror with an electrodes network.

STATE OF PRIOR ART

An ultra-fast photo-detector (response time less than 1 ps) is now acrucial element for very high-speed optical fiber telecommunications(100 Gbits/s and faster). The required performances are high sensitivityand a wide passband in the 1.3 and 1.55 μm wavelengths. Regardless ofthe type of photo-detector (PN diode, PIN diode,Metal-Semiconductor-Metal or MSM structure), the target speednecessitates a short distance between electrodes (less than 100 nm) andthat light is absorbed in a minimum volume.

Therefore existing photo-detectors require a compromise betweenefficiency and speed. Thus, the solid InGaAs semiconductor with acharacteristic absorption length of about 3 μm at the wavelength of 1.55μm, the reduction in the transit time of charge carriers is directlyrelated to a reduction in the external quantum efficiency in PIN diodesand in MSM structures.

Therefore, if existing photo repeaters were used, the increase in thespeed of optical telecommunications would require a large increase inthe number of repeaters on the transmission line and therefore in thecost of the transmission line.

Ultra-fast MSM photo-detectors (several hundred GHz) must have a thinabsorbent layer and a space between electrodes less than the wavelength.

The first constraint severely limits the sensitivity, as described inthe article “High-Speed InGaAs Metal-Semiconductor-Metal Photo-detectorswith Thin Absorption Layers” by W. A. Wohlmuth et al., IEEE Photon.Tech. Lett., Vol. 9, No. 5, 1997, pages 654 to 656.

The second constraint also severely limits the sensitivity. Theliterature describes two aspects of this constraint: firstly, anelectrode shadowing effect, and secondly a diffraction effect leading toweak penetration of light into the device. Further information aboutthis subject is given in the article entitled “High-Responsivity InGaAsMSM Photo-detectors with Semi-Transparent Schottky Contacts” by R. H.Yuang et al., IEEE Photon. Tech. Lett., Vol. 7, No. 11, 1995, pages 1333to 1335.

Research is being carried out in two different directions to overcomethis difficulty. PIN or Schottky photodiodes associated with a resonantcavity are capable of maintaining high quantum efficiency, but theircutoff frequency is limited to about 100 GHz for devices with a surfacearea of 100 μm². Further information about this subject is given in thearticle entitled “Resonant Cavity-Enhanced (RCE) Photo-detectors” by K.Kishino et al., IEEE J. Quantum Electron., Vol. 78, No. 2, 1995, pages607 to 639.

More recently, photo detectors with propagative waves have been studied.Unlike previous structures, lighting is lateral, in other wordsperpendicular to the displacement of charge carriers. These structureswere designed to act as an optical wave-guide and a TEM electricalwave-guide. The limitation due to the capacity (charge constant RC) isthen replaced by a limitation due to the mismatch between the speeds ofthe optical and electrical groups. Further information about thissubject can be found in the article “Traveling-Wave Photo-detectors” byK. S. Giboney et al., IEEE Photon. Techn. Lett., Vol. 4, No. 12, 1992,pages 1363 to 1365.

PRESENTATION OF THE INVENTION

According to this invention, it is proposed to overcome thedisadvantages of devices according to prior art by concentrating lightin a resonant manner in an MSM structure with a small active volume.Resonance is of the Fabry-Pérot type between a mirror with highreflectivity (for example a Bragg mirror) at the bottom of the deviceand a metallic electrodes network at the surface. The period of theelectrodes network is less than the wavelength of incident light.Reflectivity of this electrodes network (the second mirror in theFabry-Pérot cavity) is controlled by the geometric parameters of thenetwork. The cavity thus made comprises two parts: a thin absorbentlayer in which the photo-carriers are generated, and a non-absorbentlayer. The shortness of the paths to be followed by the photo carriersto be collected by the electrodes assures that this device has anextremely fast intrinsic behavior (response time less than onepicosecond) while resonant coupling with incident light assures a highexternal quantum efficiency (typically 10 times greater than the bestperformances at the present time).

Therefore, the purpose of the invention is an MSM type photo-detectiondevice designed to detect incident light and comprising reflecting meanssuperposed on a first face of a support to form a first mirror for aFabry-Pérot type resonant cavity, a layer of material that does notabsorb said light, an active layer made of a semiconducting materialabsorbing incident light and a network of polarization electrodescollecting the detected signal, the electrodes network being arranged onthe active layer, the electrodes network being composed of parallelconducting strips at a uniform spacing at a period less than thewavelength of incident light, the electrodes network forming a secondmirror for the resonant cavity, the optical characteristics of thissecond mirror being determined by the geometric dimensions of saidconducting strips, the distance separating the first mirror from thesecond mirror being determined to obtain a Fabry-Pérot type resonancefor incident light between these two mirrors.

The reflecting means forming a first mirror may be composed of a Braggmirror, for example composed of alternating layers of AlAs and AlGaAsand alternating layers of GaInAsP and InP or alternating layers ofAlGaInAs and AlInAs or alternating layers of AlGaAsSb and AlAsSb.

They may also be composed of a metallic layer. Preferably, the metalliclayer forming the first mirror provides a silver, gold or aluminiumsurface to incident light.

They may also be composed of a multilayer dielectric mirror.

The layer of material that does not absorb light may be made ofAl_(x)Ga_(1-x)As and the active layer may be made of GaAs. Preferably, xis of the order of 0.35 for operation at a wavelength of approximately800 nm.

The layer of material that does not absorb light may also be made ofAlInAs and the active layer may be made of InGaAs for operation at awavelength of approximately 1500 nm.

The electrodes network may form two interdigitated combs. As a variant,the electrodes network may be composed of said conducting strips thatare adjacent to each other and connected in floating potential.

Advantageously, the conducting strips are made of silver or gold.

A passive layer of dielectric material may be deposited on theelectrodes network, for example a silicon dioxide or silicon nitridelayer.

Possibly, a second face of the support supports an electrode to apply anelectrical field to the device to change the resonant wavelength of theresonant cavity by the opto-electric effect.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages andspecific features will become clear after reading the followingdescription given as a non-limitative example, accompanied by theattached drawings among which:

FIG. 1 is a partial perspective and sectional view of a photo-detectiondevice according to this invention,

FIGS. 2 and 3 are top views of an interdigitated variant of aphoto-detection device according to this invention,

FIGS. 4A to 4F illustrate the manufacture of an electrodes network andthe associated contacts for a photo-detection device according to thisinvention,

FIGS. 5A to 5C illustrate steps in a process for manufacturing anotherphoto-detection device according to this invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The structure proposed by this invention is composed of a network ofmetallic electrodes arranged on a thin layer of a semiconductingmaterial absorbing light to be detected, this thin layer being depositedon a transparent layer that is deposited on a lower mirror. Theinvention uses firstly partial transmission of incident waves (in TE andTM polarization) through the electrodes network that acts as asemi-reflecting mirror (upper mirror), and secondly resonance betweenthe upper and lower mirrors. A very large proportion of incident lightcan then be absorbed very close to the electrodes, in the active layerthat may be 50 nm thick.

The presence of a passivation layer deposited on the electrodes networkimposes a slight modification of network parameters and the distancebetween two mirrors, without affecting the existence of resonance usedin the invention.

A third electrode placed on the back face of the device uses theelectro-optic effect to change the resonant wavelength, when an electricfield is applied perpendicular to its surface, and therefore provides ameans of tuning the photo-detector to the wavelength.

FIG. 1 shows a partial perspective and sectional view of aphoto-detection device according to this invention. The photo-detectiondevice is made from a GaAs substrate 1. Reference 2 denotes the lowermirror that in this case is a Bragg mirror. This Bragg mirror iscomposed of an alternating stack of layers 21 and 22 made of materialsthat do not absorb light and have different refraction indexes. With aGaAs substrate, the Bragg mirror may be an alternation of AlAs andAlGaAs layers. The thicknesses of these layers are calculated as afunction of the range of wavelengths to be reflected. The layers 21 and22 may be obtained by epitaxy (for example epitaxy by molecular jet) onsubstrate 1.

The next step is to make successive depositions on the lower mirror 2 ofa layer 3 that does not absorb light to be detected and an active layer4. These layers may also be deposited by epitaxy. The layer 3 may bemade of Al_(x)Ga_(1-x) As with a coefficient x optimally chosen to beequal to 0.35. The active layer 4 may then be made of GaAs. It supportsa network of gold, silver or aluminium electrodes 5. The electrodes 5are composed of parallel strips at a uniform spacing. The geometricdimensions of the strips are chosen to form the upper mirror at theinterface between the electrodes network 5 and the active layer 4.

For example, for a wavelength of incident light equal to about 790 nm,the distance separating the lower and upper mirrors may be 70 nm,including 40 nm in the absorbent layer and 30 nm in the transparentlayer, the period of the electrode layer network may be 200 nm and theconducting strips may be 100 nm wide and 30 nm thick.

This structure is different from resonant MSM structures proposed in thepast because, according to the invention, bringing the electrodes closertogether no longer has the disadvantage of masking light, but also playsa fundamental role in controlling the reflectivity of the upper mirror.

If we consider the intensity of the electrical component of theelectromagnetic field in the absorbent layer for received incidentlight, it is found that absorption is very conducive to collection ofcarriers in TE polarization because it is done mainly betweenelectrodes, thus minimizing the photo-carrier collection time. In TMpolarization, the electromagnetic field is located in a region with aweak static electric field, which is not conducive to fast collection ofphoto-carriers.

The geometry of electrodes may be of the interdigitated type as shown inFIG. 2, which is a top view of a device according to the invention. Theelectrodes network is arranged on the active layer 14. It is composed ofparallel strips. 15 electrically connected to a common contact 35 toform a first comb, and parallel strips 25 electrically connected toanother common contact 45 to form a second comb. The two combs areinterdigitated. In this example, the surface covered by the electrodesnetwork is 5 μm×5 μm.

FIG. 3 is also a top view in which the device illustrated in FIG. 2 isdenoted as reference 10. It is made on a substrate reference 11. Thecontacts 35 and 45 are clearly seen. The metallic strips (references 15and 25 on FIG. 2) are not shown.

FIG. 3 shows conducting tracks 36 and 46 connecting contacts 35 and 45respectively and deposited on substrate 11. The substrate 11 alsosupports metallizations 12 and 13 surrounding the photo-detection device10 and the conducting tracks 36 and 46. The assembly composed firstly ofthe metallic tracks 12, 13 and 46, and secondly of the metallic tracks12, 13 and 36 forms coplanar lines with controlled impedance forpropagation of the photocurrent signal produced.

The electrodes can also be composed of adjacent conducting strips withfloating potential as described in document FR-A-2 803 950.

FIGS. 4A to 4F illustrate the production of an electrodes network withthe associated contacts for a photo-detection device according to thisinvention. These figures show cross-sectional views as in FIG. 1.

FIG. 4A shows a substrate 51 supporting a sequence composed of a lowermirror 52, a non-absorbent layer 53 and an active layer 54. If thesubstrate is made of GaAs, the lower mirror may be a Bragg mirrorcomposed of an alternation of AlAs and AlGaAs layers (8 or 16 pairs oflayers), the non-absorbent layer is a layer of Al_(x)Ga_(1-x)As and theactive layer is a layer of GaAs. A layer 60 made of polymethylmethacrylate PMMA with a thickness of 100 nm, was deposited on theactive layer 54. Openings are formed in the layer 60 by electroniclithography until the active layer 54 is reached in order to make theelectrodes network. Therefore, these openings may be 100 nm wide.

A silver or gold layer is then deposited on the etched PMMA layer, witha thickness of 30 nm. This silver or gold layer is represented asreference 61 on FIG. 4B. It is supported on the remaining layer 60 andon the parts of the active layer 54 that were exposed by etching.

The next step is to carry out a “lift-off” operation by dissolution ofPMMA in trichloroethylene. The result obtained is shown in FIG. 4C. Theactive layer 54 supports a network of electrodes represented as globalreference 62.

A resin layer 70 is then deposited on the active layer 54 supporting theelectrodes network 62. An optical lithography operation then etches theresin layer 70 to expose the active layer 54 at the locations providedfor the contacts associated with the electrodes. This is shown in FIG.4D.

The next step is illustrated in FIG. 4E showing a layer 71 made of thesame metal as the electrodes network, or of another metal, that wasdeposited on the remaining resin layer 70 and on the parts of the activelayer 54 exposed by etching. Layer 71 is thicker than layer 61 (see FIG.4B).

The next step is to carry out another “lift-off” operation to obtain thestructure shown in FIG. 4F. Reference 72 denotes the contacts associatedwith the electrodes network 62.

FIGS. 5A to 5C illustrate the steps in a process for manufacturinganother photo-detection device according to this invention. In thisvariant, the lower mirror is composed of a metallic layer. These figuresshow cross-sectional views as in FIG. 1.

FIG. 5A shows a semiconductor substrate 80 supporting, in sequence, astop layer 85, an active layer 84, a non-absorbent layer 83 and ametallic layer 82 designed to form the lower mirror. If the substrate 80is made of GaAs, the stop layer 85 may be made of GaInP, the activelayer 84 may be GaAs and the non-absorbent layer 83 may beAl_(x)Ga_(1-x)As. The metallic layer 82 may be a dual layer comprising afirst under-layer made of silver adjacent to the non-absorbent layer 83.The metallic layer also comprises a second under-layer that will be usedfor subsequent brazing and may, for example, be composed of gold andgermanium alloy. The lower mirror may also be a multilayer dielectricmirror.

FIG. 5B shows the previous structure fixed to a support 81.Solidarization was achieved by soldering between the metallic layer 82and a solder layer 86 deposited on the support 81. The support 81 may bemade of any material compatible with the process used.

The substrate 80 is then eliminated by mechanical, mechanochemical andchemical polishing until reaching the stop layer 85. The stop layer 85is then itself eliminated by selective chemical etching to expose theactive layer 84, as shown on FIG. 5C.

The remainder of the process, to obtain the electrodes network and itsassociated contacts, has already been described in association withFIGS. 4A to 4F.

The new MSM structure proposed by this invention comprising a resonantcavity between the Bragg mirror and a metallic “sub-wavelength” networkmade of gold, provides a means of absorbing more than 50% of incidentlight (for a wavelength of 0.8 μm) in a 40 nm thick active layer with aspace between electrodes of less than 100 nm. The efficiency is thus 25times greater than the efficiency obtained by a single passage throughthe absorbent layer. This efficiency may be significantly improved byusing a silver network, by improving the reflectivity of the Braggmirror and increasing the thickness of the absorbent layer.

The external quantum efficiency obtained on an experimental device withTE polarization is 15% in a 40 nm thick layer. There is no obvioustechnological difficulty to prevent achieving efficiencies of more than50%.

This invention may also be used for any type of photo detector using aresonant cavity. The network of metallic electrodes is used both as amirror for the cavity and a means of electrically polarizing thedetector. For example, the device may be a PIN type connector composedof a stack of p and n type semiconducting layers forming the PINjunction and incorporating an intrinsic zone (zone I), the stack beingplaced in the resonant cavity, for example composed of a Bragg mirrorand a mirror with a network of metallic electrodes, this network alsobeing used for polarization of one of the p or n layers of the junction.The device may also be a Schottky type detector composed of anon-absorbent semiconducting layer placed between a Bragg mirror and anetwork of metallic electrodes acting as a mirror for the cavity,electrical polarization electrodes and absorbent zone.

1. MSM type photo-detection device designed to detect incident light andcomprising reflecting means superposed on a first face of a support toform a first mirror for a Fabry-Pérot type resonant cavity, a layer ofmaterial that does not absorb said light, an active layer made of asemiconducting material absorbing incident light and a network ofpolarization electrodes collecting the detected signal, the electrodesnetwork being arranged on the active layer, the electrodes network beingcomposed of parallel conducting stripes at a uniform spacing at a periodless than the wavelength of incident light, the electrodes networkforming a second mirror for the resonant cavity, the opticalcharacteristics of this second mirror being determined by the geometricdimensions of said conducting strips, the distance separating the firstmirror from the second mirror being determined to obtain a Fabry-Pérottype resonance for incident light between these two mirrors. 2.Photo-detection device according to claim 1, wherein the reflectingmeans forming a first mirror are composed of a Bragg mirror. 3.Photo-detection device according to claim 2, wherein the Bragg mirror iscomposed of alternating layers of AlAs and AlGaAs d'AlAs and alternatinglayers of GaInAsP and InP or alternating layers of AlGaInAs and AlInAsor alternating layers of AlGaAsSb and AlAsSb.
 4. Photo-detection deviceaccording to claim 1, wherein the reflecting means forming a firstmirror are composed of a metallic layer.
 5. Photo-detection deviceaccording to claim 4, wherein the metallic layer forming the firstmirror provides a silver, gold or aluminum surface to incident light. 6.Photo-detection device according to claim 1, wherein the reflectingmeans forming a first mirror are composed of a multilayer dielectricmirror.
 7. Photo-detection device according to claim 1, wherein thelayer of material that does not absorb light is made of Al_(x)Ga_(1-x)Asand the active layer is made of GaAs.
 8. Photo-detection deviceaccording to claim 7, wherein x is of the order of 0.35. 9.Photo-detection device according to claim 1, wherein the layer ofmaterial that does not absorb light is made of AlInAs and the activelayer is made of InGaAs.
 10. Photo-detection device according to claim1, wherein the electrodes network forms two interdigitated combs. 11.Photo-detection device according to claim 1, wherein the electrodesnetwork is composed of said conducting strips that are adjacent to eachother and connected in floating potential.
 12. Photo-detection deviceaccording to claim 1, wherein the conducting strips are made of silveror gold or aluminum.
 13. Photo-detection device according to claim 1,wherein a passive layer of dielectric material is deposited on theelectrodes network.
 14. Photo-detection device according to claim 13,wherein the passivation layer is made of silicon dioxide or siliconnitride.
 15. Photo-detection device according to claim 1, wherein asecond face of the support supports an electrode to apply an electricalfield to the device to change the resonant wavelength of the resonantcavity by the opto-electric effect.